1. Field of the Invention
The present invention relates to an apparatus for recording an image on a recording medium.
2. Related Background Art
In already known apparatus of this kind, image signals read from a reader (or an original image reading unit) or a computer are converted into on/off signals for modulating a beam of a semiconductor laser. The modulated beam is deflected by a rotary polygon mirror having plural mirror faces to scan a rotary cylindrical photosensitive member at a constant speed in the axial direction, thereby forming an electrostatic latent image on the photosensitive member rotated at a constant speed. Image development, image transfer and image fixing are then conducted according to a known electrophotographic process to obtain a fixed image on a transported recording medium, for example a recording sheet.
The above-mentioned photosensitive member is inevitably associated with a certain irregularity in the sensitivity. More particularly in an amorphous silicon photosensitive member prepared by plasma CVD process, it is difficult to maintain the irregularity of sensitivity within 20 V. In addition an electrophotographic apparatus employing a laser optical system, for example a laser beam printer, is associated with a certain irregularity in the incident light intensity caused by interference, and a uniform electric potential has been difficult to obtain because of these factors.
In an image recording apparatus utilizing a halogen lamp or a fluorescent lamp as the light source, single or plural slits are provided in the optical path from the light source to the photosensitive member for compensating for the unevenness in the axial direction of the photosensitive member, thereby regulating the illuminating light intenstty and thus realizing a uniform potential of the latent image.
However, in an image recording apparatus utilizing a laser scanning optical system, it is technically difficult to regulate the light intensity by controlling the width of exposure, and, for this reason, it has been difficult to avoid the unevenness in the sensitivity of photosensitive member and in the incident light intensity caused by interference.
In a laser scanning optical system, reversal development is employed frequently, because the laser beam exposure in the white or background area of the original image is preferred, in comparison with the exposure in the black or character area, in reducing the light emitting period of the laser and in preventing the deterioration of resolution of the latent image, caused by a tailing effect or fluctuation of the laser spot. In such a process, however, the potential in the exposed area determines the black area of the resulting image, so that the irregularity in the sensitivity of the photosensitive member is directly reflected in the unevenness of the image density, thus deteriorating the image quality.
Also in such conventional apparatus, the surface potential of the photosensitive drum is controlled by detection of the surface potential regardless of the position of the photosensitive drum and control of the amount of charge or exposure according to thus detected value.
However such process control is often unable to provide uniform surface potential over the entire surface of the photosensitive drum, because it may have an aberration in the rotary axis or because the surface characteristic is not completely uniform over the entire surface. For these reasons it has been difficult to precisely control the surface potential of the photosensitive member by the potential control methods mentioned above. $ Also the present applicant already disclosed, in the U.S. Patent Applications Ser. Nos. 765,938, 897,053 (filed Aug. 15, 1986), 898,096 (filed Aug. 20, 1986), 900,603 (filed Aug. 26, 1986) and 010,539 (filed Feb. 3, 1987), apparatus for forming pulse-width modulated signals according to image signals, and emitting a beam in response to the signals thereby forming an image. However these apparatus are not provided with a means for correcting the characteristics of the photosensitive member.